To me bonking generally comes on fairly swift, unexpected. I generally assume I'm under fueled or dehydrated but it can also mean I just had a bad night or my body is tired. Sometimes I can fix the bonk others, no way. Generaly I start to notice the bonk when all of a sudden my legs feel heavy, I'm tired and a slight headache comes on. I slow down, maybe stop. Take in some fuel and water and attempt to end my ride as soon as possible. Best is to not bonk and take care of things before the ride.